1. Field of the Invention
The invention relates to a control device for a vehicular lockup clutch, and more particularly to improvement of flexible start control when a vehicle starts moving.
2. Description of the Related Art
In a vehicle provided with a lockup clutch that can directly connect input and output members of a hydraulic transmission device constituting part of a power transmission path between an engine and drive wheels and a linear solenoid valve serving to control an engagement hydraulic pressure for engaging and actuating the lockup clutch, a control device for a vehicular lockup clutch, which conducts flexible start control for slip-engaging the lockup clutch when the vehicle starts moving, is available. For example, such as control device for a vehicular lockup clutch is disclosed in Japanese Patent Application Publication No. 2005-016563 (JP-A-2005-016563). The control device for a vehicular lockup clutch disclosed in JP-A-2005-016563 starts the flexible start control, for example, when an accelerator is stepped on and maintains a control hydraulic pressure of the linear solenoid valve in advance at a predetermined initial engagement pressure in predetermined initial engagement section before the slip engagement of the lockup clutch is started in the flexible start control. With the control device described in JP-A-2005-016563, by performing the flexible start control in the above-described manner, it is possible to reduce a control error when slip engagement of the lockup clutch is started and perform stable slip engagement of the lockup clutch when the vehicle starts moving.
In the control device described in JP-A-2005-016563, a control hydraulic pressure of the linear solenoid valve is raised to the initial engagement pressure at the initial stage of the flexible start control, whereby the responsiveness of hydraulic control is ensured in the subsequent slip engagement of the lockup clutch. However, it cannot be said that sufficient responsiveness has been ensured. For example, when a case is considered in which the working oil used for the hydraulic control of the lockup clutch has an ultralow temperature or an ultrahigh temperature, the responsiveness will apparently be insufficient, although such data are not publicly available. A problem encountered when the responsiveness of the hydraulic control is low is that drivability and fuel consumption are degraded by an overshoot of engine revolution speed.
Further, in a fast-fill mode in which a hydraulic pressure command value for the linear solenoid valve is temporarily set high when the flexible start control is started, it is also possible to ensure the responsiveness of the engagement hydraulic pressure by setting the hydraulic pressure command value in the fast-fill mode even higher, but where the hydraulic pressure command value is set high in the fast-fill mode, a hydraulic pressure overshoot easily occurs and it is highly probable that controllability of hydraulic pressure will degrade. Such problem is not publicly disclosed.